Speaker
Description
The evolution of multicellularity has opened new evolutionary paths to increased diversity and complexity. This transition from single cells to multicellularity involved three processes: cells remained attached to one another and formed groups, cells within these new groups differentiated to perform different tasks, and the emergent groups adapted their life cycles by evolving new reproductive strategies. Recent experiments have provided insights into the selection pressures as well as the genetic toolkit that may have driven the transition from unicellular to multicellular groups and the evolution of cell differentiation. However, how the life cycles of these newly-formed multicellular groups evolved is still unknown. Here, using the budding yeast, Saccharomyces cerevisiae, as a model system, we show that alternating regimes of resource availability can select for regulated multicellular life cycles. Examining a collection of S. cerevisiae wild isolates showed that most strains exist as multicellular clusters during the haploid stage of their life cycles. We also observed that these multicellular states are strongly influenced by their environment. By genetically controlling the size of clusters, we showed that in a low-sucrose environment, previously shown to favor cooperation, cluster size (a proxy for the number of cells in an organism) directly correlated with fitness. Meanwhile, clusters are less fit than single cells when cooperation is not required (glucose) or when the environment is patchy (emulsion). Finally using both wild and engineered yeast strains, we showed that regulated life cycles have a strong advantage over constitutively single-celled or multicellular life cycles when the environment alternates between favoring cooperation and dispersal. Our results suggest that ploidy and environment regulate S. cerevisiae multicellular life cycle and that alternating resource availability may have played a role in the evolution of life cycles. We anticipate that integrating the study of wild and engineered organisms with multicellular life cycles will enhance our understanding of the factors that drove the evolution of life cycles. Specifically, combining QTL mapping of wild isolates and experimental evolution under alternating resource availability may identify the evolutionary drivers of life cycle regulation.
